Facebook's controversial paid messaging service has been spotted again. The social networking giant says the paid messages will prevent spam being sent to users of the network.

Facebook has been looking to monetise its platform for quite some time. After having launched 'pay to promote' posts feature, the social networking giant is now testing a new system for private messaging to celebrities and others outside their friends circle.

Facebook says the new system will prevent users from being bombarded with “spam” messages. It's notable the social networking site currently allows users to send messages to strangers on the network for free. However, messages to non-friends drop in the "other" inbox and are treated as spam.

The new system features a sliding scale of fees for members, based on their number of followers and how many are paying to contact them. Also, it will allow users to pay to prioritise their emails, ensuring their messages appear in the main inbox of others.

"The system of paying to message non-friends in their Facebook inbox is designed to prevent spam, while acknowledging that sometimes you might want to hear from people outside your immediate social circle, a Facebook spokesperson is quoted as saying.

"We are testing a number of price points in the UK and other countries to establish the optimal fee that signals importance. Part of that test involves charging higher amounts for public figures, based on the number of followers they have. This is still a test and these prices are not set in stone."

Facebook had started testing the paid e-mail service with select users in the U.S. earlier this year. “This test will give a small number of people the option to pay to have a message routed to the Inbox rather than the Other folder of a recipient that they are not connected with,” the company had said in a statement.